KVM Archive on lore.kernel.org
 help / color / Atom feed
From: Paolo Bonzini <pbonzini@redhat.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH] KVM: Boost vCPU candidiate in user mode which is delivering interrupt
Date: Tue, 20 Apr 2021 09:22:51 +0200
Message-ID: <b2fca9a5-9b2b-b8f2-0d1e-fc8b9d9b5659@redhat.com> (raw)
In-Reply-To: <CANRm+CyHX-_vQLck1a9wpCv8a-YnnemEWm+zVv4eWYby5gdAeg@mail.gmail.com>

On 20/04/21 08:08, Wanpeng Li wrote:
> On Tue, 20 Apr 2021 at 14:02, Wanpeng Li <kernellwp@gmail.com> wrote:
>>
>> On Tue, 20 Apr 2021 at 00:59, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>>
>>> On 19/04/21 18:32, Sean Christopherson wrote:
>>>> If false positives are a big concern, what about adding another pass to the loop
>>>> and only yielding to usermode vCPUs with interrupts in the second full pass?
>>>> I.e. give vCPUs that are already in kernel mode priority, and only yield to
>>>> handle an interrupt if there are no vCPUs in kernel mode.
>>>>
>>>> kvm_arch_dy_runnable() pulls in pv_unhalted, which seems like a good thing.
>>>
>>> pv_unhalted won't help if you're waiting for a kernel spinlock though,
>>> would it?  Doing two passes (or looking for a "best" candidate that
>>> prefers kernel mode vCPUs to user mode vCPUs waiting for an interrupt)
>>> seems like the best choice overall.
>>
>> How about something like this:

I was thinking of something simpler:

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9b8e30dd5b9b..455c648f9adc 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3198,10 +3198,9 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
  {
  	struct kvm *kvm = me->kvm;
  	struct kvm_vcpu *vcpu;
-	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
  	int yielded = 0;
  	int try = 3;
-	int pass;
+	int pass, num_passes = 1;
  	int i;
  
  	kvm_vcpu_set_in_spin_loop(me, true);
@@ -3212,13 +3211,14 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
  	 * VCPU is holding the lock that we need and will release it.
  	 * We approximate round-robin by starting at the last boosted VCPU.
  	 */
-	for (pass = 0; pass < 2 && !yielded && try; pass++) {
-		kvm_for_each_vcpu(i, vcpu, kvm) {
-			if (!pass && i <= last_boosted_vcpu) {
-				i = last_boosted_vcpu;
-				continue;
-			} else if (pass && i > last_boosted_vcpu)
-				break;
+	for (pass = 0; pass < num_passes; pass++) {
+		int idx = me->kvm->last_boosted_vcpu;
+		int n = atomic_read(&kvm->online_vcpus);
+		for (i = 0; i < n; i++, idx++) {
+			if (idx == n)
+				idx = 0;
+
+			vcpu = kvm_get_vcpu(kvm, idx);
  			if (!READ_ONCE(vcpu->ready))
  				continue;
  			if (vcpu == me)
@@ -3226,23 +3226,36 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
  			if (rcuwait_active(&vcpu->wait) &&
  			    !vcpu_dy_runnable(vcpu))
  				continue;
-			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
-				!kvm_arch_vcpu_in_kernel(vcpu))
-				continue;
  			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
  				continue;
  
+			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
+			    !kvm_arch_vcpu_in_kernel(vcpu)) {
+			    /*
+			     * A vCPU running in userspace can get to kernel mode via
+			     * an interrupt.  That's a worse choice than a CPU already
+			     * in kernel mode so only do it on a second pass.
+			     */
+			    if (!vcpu_dy_runnable(vcpu))
+				    continue;
+			    if (pass == 0) {
+				    num_passes = 2;
+				    continue;
+			    }
+			}
+
  			yielded = kvm_vcpu_yield_to(vcpu);
  			if (yielded > 0) {
  				kvm->last_boosted_vcpu = i;
-				break;
+				goto done;
  			} else if (yielded < 0) {
  				try--;
  				if (!try)
-					break;
+					goto done;
  			}
  		}
  	}
+done:
  	kvm_vcpu_set_in_spin_loop(me, false);
  
  	/* Ensure vcpu is not eligible during next spinloop */

Paolo


  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16  3:08 Wanpeng Li
2021-04-17 13:09 ` Paolo Bonzini
2021-04-19  7:34   ` Wanpeng Li
2021-04-19 16:32     ` Sean Christopherson
2021-04-19 16:59       ` Paolo Bonzini
2021-04-20  6:02         ` Wanpeng Li
2021-04-20  6:08           ` Wanpeng Li
2021-04-20  7:22             ` Paolo Bonzini [this message]
2021-04-20  8:48               ` Wanpeng Li
2021-04-20 10:23                 ` Paolo Bonzini
2021-04-20 10:27                   ` Wanpeng Li

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=b2fca9a5-9b2b-b8f2-0d1e-fc8b9d9b5659@redhat.com \
    --to=pbonzini@redhat.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=kernellwp@gmail.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=seanjc@google.com \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvm/0 kvm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvm kvm/ https://lore.kernel.org/kvm \
		kvm@vger.kernel.org
	public-inbox-index kvm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.kvm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git